1. Field of the Invention
The present invention relates to a diffusion code generator, a CDMA communication apparatus using the diffusion code generator and a method of generating a diffusion code used therefor, in particular to generation of a diffusion code in a CDMA (Code Division Multiple Access) communication apparatus.
2. Description of the Related Art
Conventionally, in a CDMA communication apparatus, a code is used to allow multiple communication in order to distinguish and divide the communication channels, which is different from a frequency in a FDMA (Frequency Division Multiple Access) system and time in TDMA (Time Division Multiple Access) system.
One of the major advantages of CDMA communication is high concealability. A factor responsible for being capable of maintaining such high concealability is highly related to length of cyclicity of a diffusion code.
Normally, a configuration as shown in FIG. 11 is used to generate this diffusion code. In FIG. 11, spreading code generator 31 is used in order to maintain orthogonality for each rate cycle of data to be diffused (information data) and usually has approximately 4 to 1024 cycles. In spreading code generator 31, a Walsh code and a hierarchical orthogonal code and so on are used.
Scrambling code generators 32 and 33 are mainly used to render power flux density on a frequency axis average or more concealable, whereby a code such as M(Maximum) length code, namely one having a cycle of 230 or so is usually used. Moreover, in FIG. 11, 34 and 35 indicate Exclusive OR (ExOR) circuits.
In addition, to distinguish between a plurality of terminals and communication channels, the same base station uses a diffusion code unique to each individual communication channel or each individual terminal and a scrambling code is usually shared. In this case, even though CDMA communication is highly concealable, interception and eavesdropping thereof become possible if the above two types of code can be identified.
Japanese Patent Laid-Open No. 11-145933 discloses a method whereby, in spread spectrum communication represented by the above CDMA communication, the type of applicable diffusion code list is cyclically or irregularly changed between spread spectrum communication apparatuses during the communication period when sending and receiving a transmitting signal itself in a diffused manner using a diffusion code in order to render the communication more concealable.
As shown in FIG. 12, this method provides a plurality (N pieces) of original diffusion code generators 41-1 to 41-N for generating different types of diffusion code list, and switches diffusion codes from these original diffusion code generators 41-1 to 41-N in response to a switching signal by selection switch 42.
On the other hand, the above CDMA communication includes the WCDMA (Wideband Code Division Multiple Access) method being considered by 3GPP (3rd Generation Partnership Project).
The 3GPP-specification structure of an uplink transport channel to a physical channel is as shown in FIG. 3, and the 3GPP-specification structure of a downlink transport channel to a physical channel is as shown in FIG. 4.
In addition, processing is performed by a procedure as shown in FIG. 5 (processes S1 to S11) in a 3GPP-specification uplink for DPCH, and processing is performed by a procedure as shown in FIG. 6 (processes S21 to S32) in a 3GPP-specification downlink for DPCH. In such a WCDMA method, a diffusion modulation using a diffusion code is performed in units of data comprising each individual slot shown in FIG. 3.
While the above-mentioned conventional CDMA communication is a system of high concealability, interception and eavesdropping thereof become possible if the diffusion code and scramble code can be identified. Thus, even CDMA communication of high concealability is highly likely to be intercepted and eavesdropped if a fixed diffusion code list is used during communication.
In addition, while the method described in the above bulletin cyclically or irregularly changes the type of applicable diffusion code list, it is not decided whether or not the type of diffusion code list is changed on a network so that there is a possibility that the type of diffusion code list after the change may overlap a type thereof used by another user.
Moreover, as the method described in the above bulletin does not take into consideration influence on the characteristics of a network such as communication quality and transmission data when cyclically or irregularly changing the type of diffusion code list, it may lead to partial elimination of data and so on along with the change of the type of diffusion code list to influence communication quality and transmission data.